A known device is disclosed in WO 01/06610 A1. An essential component in the known device is a mechanical switch which is connected to a voltage-detecting device. The two are each arranged between the two electrical lines. When the potential difference, which is detected by the voltage-detecting device, between the first and the second electrical line is too great, the switch is closed, and a short circuit is brought about between the two lines. Only as an addition to the mechanical switch are electronic switching units provided which are, however, only intended to supplement the mechanical switch. The mechanical switch containing movable parts essentially brings about the reduction in an undesired potential difference between the first and the second line.
Such devices for short-circuiting two electrical lines are often used in railroad power supply systems. In the case of a DC supply, the first electrical line is the rail, which acts as the return line, and the second electrical line is the structure ground (ground line).
In the case of isolator flashovers, potential differences between the rail and the structure ground may occur which may destroy modules which are connected to the rails.